CN113423966A

CN113423966A - Wet friction material production method

Info

Publication number: CN113423966A
Application number: CN202080013822.7A
Authority: CN
Inventors: 穆拉特·巴甘; 拉希德·法拉哈蒂
Original assignee: Schaeffler Technologies AG and Co KG
Current assignee: Schaeffler Technologies AG and Co KG
Priority date: 2019-03-25
Filing date: 2020-03-24
Publication date: 2021-09-21
Anticipated expiration: 2040-03-24
Also published as: US20200309219A1; JP2022527073A; CN113423966B; DE112020001710T5; US10844919B2; JP7309900B2; WO2020198227A1

Abstract

A method of producing a friction material. The method includes mixing silica-containing filler particles and a liquid binder to form a binder-filler liquid mixture. The method also includes saturating the fibrous base material with the binder-filler liquid mixture to form a saturated fibrous base material. The method further includes curing the saturated fibrous base material at a predetermined temperature for a predetermined time to cure the saturated fibrous base material to form the friction material.

Description

Wet friction material production method

Technical Field

Cross Reference to Related Applications

This application claims priority from U.S. application No. 16/363,685, filed on march 25 2019, the disclosure of which is incorporated herein by reference in its entirety.

The present disclosure relates generally to methods for producing wet friction materials for torque converter clutches, dual clutches, and/or transmission clutch sets, among other applications.

Background

Wet friction materials are useful for clutch applications. The wet friction material may be manufactured using a method in which a fibrous base material (e.g., pulp) and a mixture material (e.g., filler and friction modifier) are dispersed in water and then formed into paper. After the paper is dried, the formed paper may be impregnated with a thermosetting resin that is thermosetting, and then molded under pressure. The function of the wet friction material is influenced by the blending of the fibrous base material and the mixture material.

Disclosure of Invention

According to a first embodiment, a method of producing a friction material is disclosed. The method comprises the following steps: the method includes mixing silica-containing filler particles and a liquid binder to form a binder-filler liquid mixture, saturating the fibrous base material with the binder-filler liquid mixture to form a saturated fibrous base material, and curing the saturated fibrous base material at a predetermined temperature for a predetermined time to cure the saturated fibrous base material to form the friction material. The silica-containing filler particles may be present in the friction material at a concentration of 20 weight percent to 50 weight percent of the total weight of the friction material. The saturation step may be performed by immersing the fibrous base material in a bath of the binder-filler liquid mixture. The silica-containing filler particles may be present in the binder-filler liquid mixture at a concentration of 30 to 60 weight percent of the total weight of the binder-filler liquid mixture. The binder may be a phenolic resin. The silica-containing particles may be diatomaceous earth particles.

According to a second embodiment, a method of producing a friction material is disclosed. The method includes saturating a fibrous base material with a liquid binder to form a saturated fibrous base material. The saturated fibrous base material has a first surface area and a second surface area opposite the first surface area. The method further includes applying silica-containing filler particles to the first surface region and/or the second surface region of the saturated fibrous base material to form a surface region impregnated with the saturated fibrous base material. The method further comprises the following steps: curing the surface region impregnated with the saturated fibrous base material at a predetermined temperature for a predetermined time to cure the surface region impregnated with the saturated fibrous base material to form a friction material having a first concentration of silica-containing filler particles in a first friction material surface region and a second concentration of silica-containing filler particles in a second friction material surface region. The first or second concentration may be 20 to 50 weight percent silica-containing filler particles based on the total weight of the friction material. The first concentration and the second concentration each can be 20 wt% to 50 wt% silica-containing filler particles based on the total weight of the friction material. The first friction material surface region and the second friction material surface region may each have a thickness in a range of 50 μm to 150 μm. The applying step may comprise entraining the silica-containing filler particles in a gas stream to form an entrained gas stream, and applying the entrained gas stream to the first surface region and/or the second surface region of the saturated fibrous base material to form a surface region impregnated with the saturated fibrous base material. The applying step comprises placing the saturated fibrous base material and the silica-containing filler particles under vacuum conditions, and applying the silica-containing filler particles under vacuum conditions to the first surface region and/or the second surface region of the saturated fibrous base material to form a surface region impregnated with the saturated fibrous base material. The applying step includes spreading silica-containing filler particles over the first surface region and/or the second surface region of the saturated fibrous base material to form a surface region impregnated with the saturated fibrous base material. The applying step may comprise applying silica-containing filler particles to the first surface region and/or the second surface region of the saturated fibrous base material at a predetermined pressure to form a surface region impregnated with the saturated fibrous base material.

According to a third embodiment, a method of producing a friction material is disclosed. The method includes mixing a first quantity of silica-containing filler particles and a liquid binder to form a binder-filler liquid mixture. The method also includes saturating the fibrous base material with the binder-filler liquid mixture to form a saturated fibrous base material. The method further includes applying a second quantity of silica-containing filler particles to the first surface region and/or the second surface region of the saturated fibrous base material to form a surface region impregnated with the saturated fibrous base material. The method further comprises the following steps: curing the surface region impregnated with the saturated fibrous base material at a predetermined temperature for a predetermined time to cure the surface region impregnated with the saturated fibrous base material to form a friction material having a first concentration of silica-containing filler particles in a first friction material surface region and a second concentration of silica-containing filler particles in a second friction material surface region. The binder may be a phenolic resin. The silica-containing particles of the first and second quantities of silica-containing filler particles may be diatomaceous earth particles. The friction material may include a second amount of silica-containing filler particles that is greater than the first amount of silica-containing filler particles.

Drawings

The nature and mode of operation of the aspects will now be described more fully in the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a schematic cross-sectional view of a fibrous base material according to one exemplary aspect;

FIG. 2 illustrates a schematic cross-sectional view of a friction material including a filler material incorporated into the fibrous base material of FIG. 1, according to one exemplary aspect;

FIG. 3 illustrates the friction material of FIG. 2 used on a clutch plate according to one exemplary aspect;

FIG. 4 illustrates a cross-sectional view of a torque converter having friction material according to an exemplary aspect;

FIG. 5 illustrates a flow chart of a method for producing a friction material according to an exemplary aspect; and

FIG. 6 shows a graph plotting coefficient of friction versus velocity for friction material A, friction material B, and friction material C according to various embodiments.

Detailed Description

At the outset, it should be appreciated that like reference numbers appearing in different figures identify identical or functionally similar structural elements. Furthermore, it is to be understood that this disclosure is not limited to the particular embodiments, methods, materials, and modifications described herein, and as such, may, of course, vary. As one of ordinary skill in the art will appreciate, various features shown and described with reference to any one of the figures may be combined with features shown in one or more other figures to produce embodiments that are not explicitly shown or described.

The terminology used herein is for the purpose of describing particular aspects only and is not intended to limit the scope of the present disclosure, which is limited only by the appended claims. It is to be understood that the disclosed embodiments are merely examples and that other embodiments may take various alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the embodiments.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods, devices, or materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, the following exemplary methods, devices, and materials are now described.

The term "substantially" may be used herein to describe disclosed or claimed embodiments. The term "substantially" may modify a value or a relative characteristic disclosed or claimed in the present disclosure. In such cases, "substantially" may mean that the value or relative characteristic that it modifies is within ± 0%, ± 0.01%, ± 0.05%, ± 0.1%, ± 0.2%, ± 0.3%, ± 0.4%, or ± 0.5% of the value or relative characteristic.

The wet friction material may be formed by: taking a fibrous base material (e.g., pulp) and a mixture material (e.g., filler and friction modifier), and dispersing them in water and drying the dispersion to form a paper sheet, followed by impregnating the paper sheet with a thermosetting resin that is thermosetting, and molding it under pressure. This process presents challenges to many paper manufacturers. Many manufacturers do not know how to properly incorporate fillers into wet friction materials. Other manufacturers simply refuse to use fillers for wet friction materials. In the face of these challenges, it is desirable to provide a production method that: wherein paper manufacturers do not use filler materials and make it easier for entities such as automotive suppliers and original equipment manufacturers ("OEMs") to implement the process using wet friction materials. In one or more aspects, a production method and related wet friction material are disclosed that improve the efficiency of the overall process of manufacturing the wet friction material.

Referring to FIG. 1, a cross-sectional view of a fibrous base material 10 is shown. The fibrous base material 10 may be organic or inorganic fibers such as, but not limited to, cellulose fibers, cotton fibers, aramid fibers, carbon fibers, or combinations thereof. In one aspect, during the manufacture of the fibrous material, the fibrous base material 10 is made of substantially pure fibrous material by weight, except for any trace impurities that may be contained in the pure fibrous material. In one aspect, fibrous materials include cellulose fibers that are manufactured from cellulose fibers (e.g., from woody materials, non-woody materials, recycled paper, and agricultural residues) into pulp and then into paper. Pulp and paper manufacturing processes may include steps to minimize impurities in the final product. These steps may include screening, defibering and/or detangling. Similar steps can be applied in the manufacture of fibrous materials from other materials, such as cotton, aramid and carbon fibers. The amount of impurities by weight in the substantially pure fibrous material may be any one of the following values or within a range of any two of the following values: 0%, 0.01%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4% and 0.5%. In one aspect, the substantially pure fibrous material does not include any filler material. In other words, the impurities do not include any filler material.

In these aspects, the substantially pure fibrous material may be produced by a paper manufacturer and further processed by the paper manufacturer or another entity to obtain the wet friction material. In these processes, the filler material is not added during the fibrous material (e.g., pulp and paper) manufacturing process, but is added later after the fibrous material is formed.

In one aspect, the fibrous material 10 is 100% cotton fibers by weight. In another aspect, the fibrous material 10 is 100 weight percent aramid fibers. In yet another aspect, the fibrous material 10 is 100% by weight carbon fibers. Alternatively, up to 10 to 20 weight percent of the carbon fibers may be replaced with other types of fibers (e.g., cellulose fibers, cotton fibers, or aramid fibers).

The filler material (e.g., filler particles) may be arranged to carry the friction modifier and may be characterized as: (a) capable of having a surface that interacts with a friction modifier; (b) having a particle shape configured to carry a friction modifier; (c) having a particle size configured to carry a friction modifier; (d) having pores for carrying a friction modifier; or (e) any combination of (a) to (d). In one exemplary aspect, the filler material can comprise silica. In an exemplary aspect, the silica-containing particles are used to carry a friction modifier, can obtain a friction modifier, attract a friction modifier, or encapsulate a friction modifier.

The friction modifier may refer to an additive, component, or ingredient such as used in an Automatic Transmission Fluid (ATF) for an automotive component, such as a wet clutch or torque converter. In one exemplary aspect, the friction modifier is configured to provide compatibility between the plates of the metal clutch and compatibility between the ATF and the wet clutch or torque converter. The friction modifier interacts with the metal surface, with the polar head of the friction modifier binding to the clutch metal surface and the repulsive forces from the tail of the molecule, for example, contributing to the separation of the metal surface.

Typical friction modifiers include fatty amines, fatty acids, fatty amides, fatty esters, paraffins, oxidized waxes, fatty phosphates, sulfurized fats, long chain alkylamines, long chain alkyl phosphites, long chain alkyl phosphates, and boric acid-containing long chain polar (borated long chain polar). In one exemplary aspect, the friction modifier includes a generally pure oleophilic tail comprising 10 to 24 carbons and an active polar head group portion. In another exemplary aspect, the tail comprises 18 to 24 carbons. The head forms a layer on the friction surface by surface adsorption. The friction modifier is configured to not corrode or cause decomposition of the filler material or clutch plates, which are typically made of steel. A non-limiting example of a friction modifier that may be used in one exemplary aspect is stearic acid.

In one exemplary aspect, the filler material comprises silica-containing particles. The silica-containing particles may carry a friction modifier, may obtain a friction modifier, attract a friction modifier, or encapsulate a friction modifier. In one exemplary aspect, the silica-containing particles may be diatomaceous earth particles (DE). DE is a source of natural silica formed by the settlement of unicellular aquatic organisms known as diatoms. DE can form in seawater or freshwater environments and exhibit properties related to its unique shape and structure. These characteristics will vary according to the diatom species found in each deposit, each with a different chemical compositionMinute, form factor and pore structure. Some non-limiting examples of silica-containing support particles include

And CelTiX^TM。

Is fluxing calcined diatomite of plankton marine diatomite.

Is natural diatomite material. CelTiX^TMIs a superior natural freshwater diatomite product with excellent reinforcement capabilities in most types of elastomers. Silicon dioxide (Silica) is also known as silicon dioxide (Silica dioxide) or SiO₂. Diatomaceous earth typically contains about ten percent of other oxides in addition to silica, and is substantially free of crystalline silica. Typically, diatomaceous earth is amorphous.

FIG. 2 shows a schematic cross-sectional view of a friction material 12 incorporating a filler material 14 incorporated into a fibrous base material 10. As described below, one or more aspects of the method of producing a friction material may be used to incorporate the filler material 14 into the fibrous base material 100. As shown in FIG. 2, the friction material 12 includes a first surface 16 and an opposing second surface 18. The friction material 12 also has a first surface area 18 defined by the first surface 16 and a first depth 22 having a first thickness therebetween. The friction material 12 also has a second surface area 24 defined by a second surface and a second depth 24 having a second thickness therebetween. The friction material 12 also has a body region 28 defined by the first depth 22 and the second depth 26 having a body thickness therebetween. The first thickness and the second thickness may be independently selected from any one of the following values or within a range of any two of the following values: 50 μm, 75 μm, 100 μm, 125 μm and 150 μm. The body thickness may be selected from any one of the following values or within a range of any two of the following values: 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm and 1.0 mm. As shown in fig. 2, the concentration of filler material 12 in the first surface region 20 and/or the second surface region 24 is higher than the concentration of filler material 12 in the body region 28. The weight percent of filler material 12 in the first surface region 20 and/or the second surface region 24 may be independently selected from either or within a range of any two of the following values: 20 wt%, 30 wt%, 40 wt% and 50 wt%. The weight percent of filler material 12 in the body region 28 may be any one of the following values or within a range of any two of the following values: 1 wt%, 5 wt%, 10 wt%, 15 wt% and 20 wt%.

As shown in fig. 3, the friction material 100 is fixedly secured to the clutch plate 102. The friction material 100 includes a fibrous base material 104 that includes a filler material 106. As described below, one or more aspects of the method of producing a friction material may be used to incorporate the filler material 104 into the fibrous base material 104. The friction material 100 may also include a binder such as phenolic, latex, silane, and mixtures thereof. The fibrous base material 104 may be organic fibers or inorganic fibers. Non-limiting examples include cellulosic fibers, cotton fibers, aramid fibers, carbon fibers, or combinations thereof. The friction material 100 has a high concentration of filler material 106 at a first surface area 108 of the friction material 100.

FIG. 4 illustrates a cross-sectional view of a torque converter 200 having friction material 12, friction material 100, according to an exemplary aspect. Torque converter 200 includes a cover 202, an impeller 204 connected to cover 202, a turbine 206 in fluid communication with impeller 204, a stator 208, an output hub 210 arranged to non-rotatably connect to an input shaft (not shown) for the transmission, a torque converter clutch 212, and a damper 214. The clutch 212 includes the friction material 12, the friction material 100, and a piston 216. The piston 216 is displaceable to engage the friction material 12, the friction material 100 with the piston 216 and the cap 202 to transfer torque from the cap 202 to the output hub 210 through the friction material 100 and the piston 216. Fluid 218 is used to operate clutch 212. The friction material 12, friction material 100 may be used in any clutch device for any torque converter configuration.

FIG. 5 depicts a flow chart of a process 300 that may be used to produce the friction material 12 and/or the friction material 100. As shown in fig. 5, process 300 includes a plurality of

steps

302, 304, 306, 308, and 310. In certain aspects, one or more of these steps may be modified or omitted depending on the implementation.

In step 302 of process 300, the binder material is dissolved in a solvent. In one aspect, a solid binder material (e.g., pellets of a solid binder material) is dissolved in a solvent to form a liquid binder material. The liquid bonding material may be held in a container or transferred to a different container for further processing, including one or more of the other steps of process 300. Typically, the binder material is a phenolic resin. Upon curing, the phenolic resin forms water as a by-product of the reaction between phenol and formaldehyde.

Are non-limiting examples of phenolic resins that may be used with the friction material.

In step 304 of process 300, filler particles are mixed with a liquid binder material to form a liquid mixture of filler particles and liquid binder. The filler particles can be any material or mixture of materials as described herein in one or more aspects, such as silica-containing particles (e.g., diatomaceous earth particles). The liquid binder material may be any material or mixture of materials as described herein in one or more aspects, such as a phenolic resin. In one exemplary aspect, the filler particles are substantially homogeneously mixed with the liquid binder material. In another aspect, the filler particles are substantially non-uniformly mixed with the liquid binder material. In one aspect, the filler particles are present in the binder-filler liquid mixture at a concentration of 30 to 60 weight percent of the total weight of the binder-filler liquid mixture.

In step 306 of process 300, the fibrous base material is saturated with a liquid mixture of filler particles and liquid binder. The fibrous base material may be any material or mixture of materials as described herein in one or more aspects. In one example, step 306 may be performed by immersing the fibrous base material in a bath of the binder-filler liquid mixture. The immersion process may be carried out in a continuous manner. In one aspect, a roller or pair of rollers may be used to remove excess liquid mixture from the fibrous base material as saturated fibrous base material emerges from the bath. In one aspect, step 308 may be performed between

steps

306 and 310. In another aspect, step 308 may not be performed between

steps

306 and 310.

In step 308 of process 300, filler particles are applied to the first surface region and/or the second surface region of the saturated fibrous base material to form a surface region impregnated with the saturated fibrous base material. The first surface area may be opposite the second surface area. The thickness of the first surface region and the second surface region may be any thickness as described herein. For example, the first thickness and the second thickness may be independently selected from any one of the following values or within a range of any two of the following values: 50 μm, 75 μm, 100 μm, 125 μm and 150 μm. The concentration of filler particles in the first surface region and/or the second surface region after step 308 may be independently selected from the weight percent of filler material within the range of either or both of the following values: 20 wt%, 30 wt%, 40 wt% and 50 wt%. In one aspect, the application of filler particles in step 308 is an alternative to adding filler particles to the liquid binder to saturate the fibrous base material. In another aspect, the application of the filler particles in step 308 is in addition to the filler particles in the liquid binder that saturates the fibrous base material.

The applying step 308 may include entraining the filler particles in a gas stream (e.g., an air stream) to form an entrained gas stream that is applied to the first surface region and/or the second surface region of the saturated fibrous base material. The applying step 308 may include placing the saturated fibrous base material and filler particles under vacuum conditions and applying the filler particles to the first surface region and/or the second surface region of the saturated fibrous base material under vacuum conditions. The applying step 308 may include dispersing filler particles over the first surface area and/or the second surface area of the saturated fibrous base material. The applying step 308 may include applying filler particles to the first surface region and/or the second surface region of the saturated fibrous base material at a predetermined pressure. For example, the saturated fibrous base material may be compressed by a pair of rollers (e.g., compression rollers). The predetermined compression may be any one of the following values or within a range of any two of the following values: 20%, 25%, 30%, 35% and 40%. In one example, the saturated fibrous base material may have a thickness of 1.0mm and the roll gap may be 0.7 mm. In this example, the percentage of compression may be 30%. In one aspect, the filler particles may be applied to a roll or pair of rolls (e.g., pressure rolls) used to drive off excess liquid binder from the saturated fibrous base material.

In step 310 of the process 300, the saturated fibrous base material is cured at a predetermined temperature for a predetermined time to cure the saturated fibrous base material to form the friction material. The predetermined temperature may be any one of the following values or within a range of any two of the following values: 150 ℃, 155 ℃, 165 ℃, 175 ℃, 185 ℃, 195 ℃ and 200 ℃. The predetermined time may be any one of the following values or within a range of any two of the following values: 3.5 minutes, 4 minutes, 4.5 minutes, and 5 minutes. After step 310, the filler particles may be present in the friction material at a concentration of 20 to 50 weight percent of the total weight of the friction material. In one aspect, step 308 may result in a friction material having a first concentration of filler particles in a first surface region and a second concentration of filler particles in a second surface region. The first concentration and/or the second concentration may be 20 wt% to 50 wt% of the filler particles based on the total weight of the friction material.

FIG. 6 shows a graph 400 plotting coefficient of friction at 120 ℃ at a surface pressure of 3MPa versus velocity for friction material A, friction material B, and friction material C, according to various embodiments.

Curves

402A, 402B and 402C for friction material a, friction material B and friction material C show sufficient friction performance under the aforementioned conditions. The friction material a has a fibrous base material composed of 100 wt% of aramid fibers. The friction material B has a fibrous base material composed of 50 wt% of cellulose fibers and 50 wt% of aramid fibers. The friction material C has a fibrous base material composed of 100% by weight of cellulose fibers. The process 300 (including

steps

302, 304, 306, 308, and 310) is used to incorporate filler materials for the diatomite particles into each of the friction materials. The weight percentage of the diatomite particles in each friction material was about 30 wt% based on the total weight of each friction material.

While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure. As previously mentioned, features of the various embodiments may be combined to form further embodiments of the invention, which are not explicitly described or shown. Although various embodiments may have been described as providing advantages in one or more desired characteristics or over other embodiments or prior art embodiments, one of ordinary skill in the art will recognize that one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the particular application and embodiment. These attributes may include, but are not limited to, cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, and the like. As such, to the extent that any embodiment is described as having a lower desired value than other embodiments or prior art embodiments with respect to one or more characteristics, such embodiments do not depart from the scope of the present disclosure and may be desired for particular applications.

Parts list

The following is a list of reference numerals shown in the drawings. However, it should be understood that the use of these terms is for illustrative purposes only for one embodiment. Also, the use of reference signs placed in relation to certain terms, as used herein, and as shown in the claims, is not intended to limit the claims to cover the illustrated embodiments only.

Fibrous base material 10 friction material 12 filler material 14 first surface 16 second surface area 20 first depth 22 second surface area 24 second depth 26 body area 28 fibrous base material 104 filler material 106 first surface area 108 torque converter 200 cover 202 impeller 204 turbine 208 output hub 210 torque converter clutch 212 damper 214 piston 216 fluid 218 process 300 step 302 step 304 step 306 step 310 graph 400 curve 402A curve 402B curve 402C.

Claims

1. A method of producing a friction material, the method comprising:

mixing silica-containing filler particles and a liquid binder to form a binder-filler liquid mixture;

saturating a fibrous base material with the binder-filler liquid mixture to form a saturated fibrous base material; and

curing the saturated fibrous base material at a predetermined temperature for a predetermined time to cure the saturated fibrous base material to form a friction material.

2. The method of claim 1, wherein the silica-containing filler particles are present in the friction material at a concentration of 20 to 50 weight percent of the total weight of the friction material.

3. The method of claim 1, wherein the saturating step is performed by immersing the fibrous base material in a bath of the binder-filler liquid mixture.

4. The method of claim 1, wherein the silica-containing filler particles are present in the binder-filler liquid mixture at a concentration of 30 to 60 weight percent of the total weight of the binder-filler liquid mixture.

5. The method of claim 1, wherein the binder is a phenolic resin.

6. The method of claim 1, wherein the silica-containing filler particles are diatomaceous earth particles.

7. The method of claim 6, wherein the diatomaceous earth particles are amorphous.

8. The method of claim 1, wherein the saturated, fibrous base material has a first surface area and a second surface area opposite the first surface area.

9. The method of claim 8, further comprising applying the silica-containing filler particles to the first surface region and/or the second surface region of the saturated fibrous base material to form a surface region impregnated with saturated fibrous base material.

10. The method of claim 9, further comprising curing the surface region impregnated with the saturated fibrous base material at a predetermined temperature for a predetermined time to cure the surface region impregnated with the saturated fibrous base material to form a friction material having a first concentration of the silica-containing filler particles in a first friction material surface region and a second concentration of the silica-containing filler particles in a second friction material surface region.